Field of the Invention
This invention relates to semiconductor devices and more particularly, to lead materials for use in semiconductor devices such as transistors, integrated circuits, and the like.
As is well known in the art, it is the usual practice to use, as lead materials for semiconductor devices, Fe-42%Ni which has a linear expansion coefficient close to the coefficients of elements and ceramic materials. This Fe-42%Ni alloy has such a large content of Ni that it is poorer in economy than copper alloys.
In the fabrication of integrated circuits, it is essential to attain high cost performance. Thus, inexpensive copper alloys have been studied and developed as a substitute for Fe-42%Ni. In fact, the problem of the thermal stress produced due to the difference in linear expansion coefficient between an element and a lead is now solved by techniques such as insertion of a thin sheet of tungsten or molybdenum as a cushioning medium, or use of solders or conductive resin adhesives. In addition, although expensive ceramics have been conventionally used as adhesives and sealed with glass, they are now replaced by inexpensive resins in order to save the cost. Resins are advantageous in that they have linear expansion coefficients close to the coefficients of copper alloys and show good affinity for the alloys.
As the degree of integration increases, Fe-42wt%Ni has to be so designed that a heat sink is provided in order to diffuse the Joule heat generated in an element. However, such a heat sink is not necessary for copper alloys.
As will be appreciated from the above, copper alloys have great advantages when used, instead of conventional Fe-42wt%Ni, as the lead material for semiconductor devices. It should be noted, however, that Fe-42wt%Ni is advantageous in that they have high strength, great elongation, and good heat resistance.
In general, lead materials for semiconductor devices should satisfy the requirements of high strength, good lead bend fatigue property, high stiffness, good heat and electrical conductivity, good corrosion resistance, good resistance to stress corrosion cracking, good solderability, good resistance to peeling of plated tin and solder, good platability of gold or silver, good stampability, and inexpensiveness.
Among copper alloys suitable as lead materials for semiconductor devices, Cu-Ni-Si alloys are considered as having mechanical properties and heat resistance almost same as Fe-42wt%Ni. However, the alloys are poorer in hot workability and is also disadvantageous in that plated tin or solder is peeled in several days when maintained at a temperature of 150.degree. C. and that platability of gold or silver is worse.